The RPE cell plays a basic role in maintaining the structural and physiological integrity of the neural retina. Alterations in its structural and functional actions can result in loss of photoreceptors and vision. We have studied the RPE cell extensively as an important immunoregulatory cell within the posterior pole of the eye. Our research activities on RPE cells can be subdivided into three categories: normal cell function studies, cytokine interactions and infectious processes. This project has concentrated on studying the ways in which cytokines interact with cells of the immune system and with cells in the ocular microenvironment. These studies indicate that cytokine-mediated activation of RPE cells may be a basic component of ocular immunity and an important aspect of RPE cell transplantation. During the past year, we have studied the Toll-Like receptors (TLR), production of platelet derived growth factors (PDGF), and ATP-binding cassette (ABC) in RPE cells. TLRs are crucial components of innate immunity that participate in host defense against microbial pathogens. We evaluated the expression and function of TLRs in human retinal pigment epithelial (RPE) cells. Real time PCR analysis revealed gene expression for TLR 1 to 7, 9 and 10 in RPE cells. TLR 1 and 3 were the most highly expressed TLRs. TLR 3 is the receptor for dsRNA, an intermediate of virus replication. Since RPE cells express TLR 3 and are frequently the site of virus replication within the retina, we evaluated TLR 3 signaling. RPE cells treated with poly I:C produced IFN-beta but not IFN-alpha, and this was inhibited by treatment of RPE cells with anti-TLR 3 antibody. Human recombinant IFN-beta was shown to be biologically active on RPE cells by inhibiting viral replication. Poly I:C treatment of RPE resulted in an increase in the production of IL-6, IL-8, MCP-1 and sICAM-1. The presence of TLRs on RPE cells and the resultant TLR signaling in RPE cells suggest that these molecules may play an important role in innate and adaptive immune responses within the retina. In our continued studies on cytokine interactions in RPE cells we observed that platelet derived growth factor (PDGF) gene expression and protein production was up-regulated in response to TGF-beta treatment. These studies indicate that the promotion of the proliferation and migration of mesenchymal cells by RPE cell derived PDGF may facilitate the formation of fibrovascular tissues associated with proliferative vitreo-retinopathy (PVR). ATP-binding cassette (ABC) transporters play a pivotal role in physiology and pathology. In collaborative studies we identified two mRNA isoforms (ABCB5a and ABCB5b) are expressed in normal melanocytes and in RPE cells. These studies suggest that ABCBa/b expression is pigment cell-specific and might be involved in melanogenesis.